I'll try to cover this topic as much as possible. If you already know of a certain item, cool!
; if not, I hope this text
makes sense and you are able to learn from it.
Also, others curious to the topic may benefit from a somewhat complete description.
*** Terms, Acronyms and Abbreviations used:
* VAC = Volts, Alternating Current,
* LCL = Long, Continuous Load(s),
* Current = Amperes, Amps,
* MCCB = Molded Case Circuit Breaker,
* OCPD = Over Current Protective / Protection Device,
* SCA = Short Circuit Amperes,
* G.P. = General Purpose,
* DED = Dedicated,
* HID = High Intensity Discharge Lighting,
* Spec. Ckt. = Specific Load Circuit,
* Trip Rating = Rating, in Amperes, for a given OCPD (i.e. 20 Amp circuit breaker is 20 Amp trip rated),
* THD = Total Harmonic Distortion,
* SMPS = Switch-Mode Power Supply.
To start things off, the definition of Non-Continuous and Continuous Loads should be covered.
* Continuous Load: A Load that is drawing a fixed level of Current for 3 Hours or More (180 Minutes or more).
* Non-Continuous Load: A Load drawing a fixed level of Current for less than 3 Hours (<180 Minutes).
Using the above example, take a given load, run it for 179 minutes then turn it off for 30 minutes, then turn it back on
for 179 minutes, etc... and this will be a Non-Continuous Load. Although this is a very crude example, it makes the
necessary points of a Non-Continuous load.
Same load run for 180 minutes, turned off, run once again for 180 minutes (or even just run once for 180+ minutes), is
now a Continuous load.
Next, for Circuitry (and load calcs + Panel Schedule calcs), any load that is classified as a Continuous Load will
receive an "LCL Adder". This means the Load Current on a Continuous Load receives an additional 25% rating,
making it 125% of the actual load Current.
Example: a Continuous Load draws 10.0 Amperes. With the LCL adder, the load is now figured as 12.5 Amperes -
even though there will only be 10.0 Amperes drawn by the Load.
LCL is only figured once - meaning that if the Branch circuit having the Continuous Load is calculated at 125% (I ×
1.25), this is the only place to figure LCL. The LCL will be "Automatically" carried along with the Subfeeders, to the
next Panel, and so on.
If the Branch Circuit is not figured with LCL added, then it may be added to the Subfeeder. Once again, it only needs
to be added once, but it gets carried over through the system.
FYI: It's better to figure the Branch circuit with LCL, so proper calcs may be done to the given circuit.
These calcs are for System Performance and overall Equipment rating, so the equipment may handle the Continuous
Main item of concern here (and for any LCL Load) is HEAT!!! I²R losses increase with heat! Component failure
increases with heat! Insulation breakdown situations increase with heat!
Heat is the key "to avoid" item in all Electrical Designs and Installations.
Now, for the OCPD and the "80%" thing:
Generally speaking, most of the MCCBs dealt with in normal Construction Electrical Installations are "Not 100%
A "100% Rated Frame" will be able to carry a Continuous Load Current for >3 Hours, which is at the rating of the
device's Maximum, without "Derating" the circuit to "80%" of the Maximum.
On "Common", non-100% frames, if an LCL circuit is run through it, the LCL load can only be 80% of the device's
trip rating (current rating). Remember when I said to only add LCL once, this also applies here.
Examples of OCPD loads with and without LCL - for standard types and 100% rating types:
Loads will be 20.0 Amps steady.
Using a "Standard" rated type frame (non-100%):
20 Amp load on 20 Amp circuit (breaker + #12 THW / THHN cu) for <180 Minutes: No Derating needed.
20 Amp Load for 180 minutes or more: Derate circuit to 80% Capacity.
This results in either reducing the total load to 16 Amps (20 × 0.8 = 16) and using a 20 Amp circuit (20 Amp breaker
+ #12 THW / THHN cu),
Installing a 30 Amp circuit for the 20 Amp continuous Load (30 Amp breaker + #10 THW / THHN cu).
The 30 Amp circuit has a Maximum of 24 Amps for a Continuous Load.
In this design situation, it would help to figure the branch circuit with LCL added in first, then size circuit per the total
Using a 100% Rated Frame:
Circuit may be driven with a Continuous Load of 20 Amps for >180 minutes, and still use 20 Amp circuit.
In this design situation, best to NOT figure branch circuit with LCL first (need to know the "real" continuous load
current), add the LCL to the Feeders instead.
As to the trip rating, here's some information (Standard Overload only, not the AIC ratings or the Time-Current curve
figures...these are way beyond the scope of this discussion):
A circuit breaker / fuse may have 100% of it's rating drawn across it for eons, yet it will not trip (unless the breaker is
weak and sucks really bad!). If excessive heat is concentrated within the Panelboard, this may effect the maximum trip
points of breakers - making them trip at levels below their maximum rating - like 85-90% of the trip rating.
Trip ratings are for 100% of the rating, at a given system VAC.
Example: a Square D QOB 320 (3 pole 20 amp 240 VAC max) can carry 20 Amps for an infinite time, and will not
trip under normal circumstances.
The same breaker may allow 21 amps to flow across it for a few days before it trips (if it trips at all!).
Same breaker may allow 25 Amps for 3-5 hours, 30 amps for 1-3 hours, 40 amps for upto an hour, 60 amps for 15
minutes, 100 amps for 1 - 5 minutes, and so on. (this is the Time-Current trip Curve).
Under fault situations, the same breaker may trip in 10 seconds with a SCA of 300 amps, 5 seconds with an SCA of
500 amps, and never trip (even explode) with an SCA of 25,000 amps. (this is the AIC rating and figures of
Time-Current trip too).
Now to applications!
Mainly, an LCL load is kind of obvious. Lighting is one obvious one. Certain Ded. and Spec. Ckt. loads may fall into
the LCL realm.
For HID Lighting, there is also the need to figure starting current in the circuit, for certain Ballast types. On Fluorescent
Lighting using high Frequency Electronic Ballasts with THD >10%, the circuitry needs to be adjusted accordingly for
the excessive Harmonic Load Current.
Same goes for Computer equipment and other equipment using SMPSs.
Along with the Ungrounded Conductors having an LCL and Harmonic load calc, the Common Grounded Conductor -
when used - should also get equal calcs.
On G.P. Receptacle circuits, it's very difficult (actually impossible) to determine what LCL devices might be used.
At times, it may be "Assumed" - like portable floor heaters, but this is such a variable.
Proper System designing (along with Installation) would reflect the Client's needs and load requirements.
I'll finish this off here, since it's running very long!
Let me know if this was helpful, if something is questionable, if something needs to be edited, or if I should add more
on the subject later.
p.s. feel free to bring up another topic for discussion!